1. Field of Invention
This invention relates generally to a display whose appearance depends both on the angular position of the light source illuminating the display and the angular position of the observer relative to the display, and more particularly to a display adapted to present selectively to the observer colors and images that depend on said angular positions.
2. Status of Prior Art
Retroreflectivity is the term applied to reflection wherein reflected light rays return along paths parallel to those of the corresponding incident rays. This phenomenon is often exploited in highway signs that function to reflect light projected by automobile headlights directly back to the driver of the vehicle.
Thus the White U.S. Pat. No. 2,251,386 discloses a sign in which a layer of glass beads is applied to a painted plate, the beads being in a formation representing a curve in the road. The glass bead layer has retroreflective properties, hence when illuminated by a headlight beam, the driver of the auto sees the illuminated symbol of a curve in the road. In the highway STOP sign shown in the Meigs U.S. Pat. No. 2,411,222, the letters of STOP are each defined by a layer of glass beads, so that when the sign is illuminated, the driver of the vehicle then sees STOP.
It is also known to combine retroreflectivity with color. Thus the Palmquist U.S. Pat. No. 2,383,884 shows a reflector in which a layer of glass beads is adhered by a binder to a colored translucent film attached to a metal reflector to provide colored reflex light reflector sheets and signs. Boora et al. U.S. Pat. No. 3,877,786 discloses a reflex reflective device in which a layer of glass beads is combined with a transparent layer of a color receptive resin. Thus it is known to combine retroreflectivity with color.
In a display in accordance with the invention, the phenomenon of retroreflectivity is combined with that of goniochromatism to selectively produce color and image effects that depend on the angular position both of the light source illuminating the display and the angular position of the observer who views the display.
Goniochromatism is a phenomenon encountered when a surface is coated with luster pigments having light interference properties, such as those disclosed in the Pigment Handbook, Vol. 1, 2nd Edition 1988, John Wiley & Sons, Inc. As pointed out in this Handbook, pearlescent or nacreous pigment particles are transparent platelets of high refractive index which partially transmit light. When the reflecting platelets are in an appropriate thickness, the nacreous pigment then behaves as an interference pigment exhibiting a color play that verges on irridescence.
Light interference arises through interactions between reflections from the upper and lower surfaces of the platelets. When an interference pigment is coated on a white surface, he reflection color is seen in the highlight and the transmission color in the background. The variations in color with the angle of incidence and the angle of observations is referred to as goniochromatism. Hence the color one sees when looking at a goniochromatic layer varies with changes in angular position relative to this layer.
Interference pigments such as those formed from titanium dioxide-coated mica, iron oxide-coated mica and bismuth oxychloride are commercially available from the Mearl Corporation of New York, N.Y. and from other sources.
Inasmuch as the present invention resides in combining retroreflectivity with goniochromatism to create multiple images and changes off colors, of background interest are known types of multiple image techniques. In depthography, two or more images are applied to the back side of a plastic or glass sheet whose front face is etched with fine parallel V-shaped grooves. As a consequence, one image is visible when viewing the sheet at one oblique angle while the other is rendered visible when the sheet is viewed at another angle. This technique is often used in such items as picture postcards depicting two views of a given subject.
Another known process is holography in which a light beam from a laser is split, one half beam being reflected from the subject being photographed and being combined with the other half beam on a photosensitive surface to produce three-dimensional image through light interference. The multiple-image holography process is complex and costly and is limited commercially to relatively small sizes.